1. Field of the Invention
The present invention relates to a liquid crystal projector which is excellent in light stability.
2. Description of the Related Art
As a liquid crystal projector using a reflective type liquid crystal panel, for example, the display device having a high-density reflective type TFT-LCD has been reported (Y. Takubo et. al.,:Japan Display '89 p. 584 (1989)). This display device provides a reflective type liquid crystal panel which is arranged to form a pixel electrode made of a TFT on a TFT element through an insulated material for improving an aperture ratio and serving itself as a reflective panel.
The liquid crystal projector uses an optical addressed liquid crystal light valve. The optical addressed liquid crystal light valve is composed of a liquid crystal layer for modulating a reading out light according to change of a voltage, a light reflective layer for reflecting the reading ray, a light blocking layer for blocking a light transmitted from the light reflective layer, and a photoconductive layer for changing impedance according to the intensity of an incident light for controlling a voltage applied onto the liquid crystal layer, all of these components being sandwiched between transparent substrates made of glass. To drive the device, at first, an s.c. voltage is applied to the device. If no light (writing light) from the transparent substrate comes to the photoconductive layer, the bias voltage is mainly applied to the photoconductive layer. Further, if the writing light is applied to the device, the photoconductive layer lowers its impedance so that almost of the bias voltage may be applied to the liquid crystal layer, thereby modulating the reading light.
The operating mode used in the conventional liquid crystal light valve is a hybrid field-effect mode (abbreviated as a HFE mode) at which a nematic liquid crystal layer with positive dielectric anisotropy is twisted by 45.degree.. If the voltage is applied onto the liquid crystal layer, the liquid crystal molecules respond to an electric field and tilt toward the vertical of the substrate. The polarizing direction of the incident light is rotated through the birefringent effect and the reflection caused by the tilted and twisted liquid crystal molecules. The ray whose polarization is rotated is transmitted through the polarizing beam splitter, so that the screen may be in a bright state. Considering that no voltage is applied, if a reading out light is reflected through the effect of a optical rotary power held by the liquid crystal, the direction of polarization provided at the ray incident time is kept constant so that the screen may be in the dark state. That is, assuming that an anisotropy in an index of refraction of the liquid crystal is .DELTA.n, a thickness of the liquid crystal layer is d, and a wavelength of an incident light is .lambda., the reflectance characteristic provided when the HFE mode in which applied voltage is off is made zero if cos (2.pi..DELTA.n.d/.lambda.)=1 (William. P. Bleha, Jan Grinberg, Alexander D. Jacobson and Gary D. Myer Hughes Research Laboratories, Malibu, Calif.:SID '77 Digest P. 104). That is, the reflectance characteristic at the OFF time depends on the wavelength. It will be understood from this fact that it is necessary to select the values of .DELTA.n, d and .DELTA. in a manner to establish the relation of (.DELTA.n.d/.lambda.)=K (K is an integer) when designing the panel.
Further, as an operating mode of the liquid crystal display, it is possible to use a twisted nematic (TN) mode using nematic liquid crystal, a super twisted nematic (STN) mode, an electrically controlled birefrigence (ECB) mode, or a surface stabilized ferro electric liquid crystal (SSFLC) mode using ferroelectric liquid crystal. The ECB mode may be divided into three types based on the molecular orientation in the initialized state. Of these types, the deformation of vertical aligned phase type ECB mode keeps the liquid crystal vertically oriented in the initial state, because the nematic liquid crystal with a dielectric anisotropy is used. The electric field serves to tilt the liquid crystal molecules so that the birefrigence caused by the tilted molecules may cause the display to enter into a bright or a dark state. In the case that the liquid crystal molecules keep their orientation completely vertical, if any voltage is applied to the liquid crystal, the liquid crystal molecules are tilted at random, resulting in remarkably lowering the display quality. Hence, it is necessary to develop a technique of orienting the liquid crystal molecules widely over the glass substrate as keeping a uniform pre-tilt angle. If the pre-tilt angle is large, the birefringent effect serves to penetrate a light through the liquid crystal layer in the initial state, thereby lowering the contrast. It means that the pre-tilt angle has to be fine.
Some papers have reported techniques of keeping the liquid crystal in the tilted vertical orientating state.
The substrate is tilted by 85.degree. (deposition angle o=85.degree.) against the vertical of the substrate laid horizontally so that silicon oxide (SiO.sub.x) may be deposited on the substrate in vacuum (oblique deposition). The thickness of the silicon oxide is about 70.ANG.. The liquid crystal cell created by this method keeps the same orientation as the cell which was subject to the rubbing process. (J. L. Janning. Applied Physics Letter 21 p. 173 1972) Further, a heat cycle test (a reliability test executed to repeat a cycle of a room temperature to 250.degree. C. to a room temperature 100 times) is executed for the cell created by the oblique deposition before or after injecting the liquid crystal. This results in keeping the orientation of the liquid crystal stable (W. Urbach et, al. Applied Physics Letter 25 p. 479, 1974). However, nothing about light stability is described in the papers.
There has been proposed a method in which after creating an inorganic film on the substrate by obliquely depositing SiO.sub.x, the finishing is carried out. In this method, as a surfactant, CTAB (Cethyl-trimethyl-ammonium bromide) is used. The CTAB film is created by slowly and vertically pulling up the substrate from the surface of the CTAB solution. In the case that a CTAB density in the CTAB solution is sufficiently high, the vertical orientation is kept in the relation of o&lt;40.degree. (W. Urbach et, al. Applied Physics Letter 25 p. 470 1974). However, since the CTAB is not chemically attached on the surface of SiO.sub.x, it is well known that some or all of the CTAB is solved in the liquid crystal depending on a liquid crystal structure, a thickness of a liquid crystal layer and a temperature (Patent Lying Open No. Hei 2-503482 applied by the Hughes Aircraft Company). Hence, it is not expected that the CTAB formed on the surface or the CTAB solved into the liquid crystal is excellent in light stability under the high illumination.
Further, retisine or "T" acid is well known not as a surfactant agent but as a dopant to the liquid crystal (K. Fahrenschon and M. F. Schiekel Journal of Electrochemistry Society 124 p. 953 1977). By depositing SiO or magnesium fluoride (MgF.sub.2) on the glass substrate in vacuum and at a depositing angle of 60.degree. to 85.degree. for forming a cell, the pre-tilt angle (.theta.p) of 12.degree. to 16.degree. can be obtained. However, there is no description about light stability. In the following example, the finishing is done by using DMOAP (N, N dimethyl-N-octadecy 1-3-aminopropyltrimethoxysilil chloride) and a UTPFE (Ultrathin polyfluoreoethylene) film on the surface on which SiO is deposited at an angle of o=85.degree. (W. R. Heffner et, al. Applied Physics Letter 36 p. 144 1980). The material of DMOAP can be obtained by dipping a DMOAP solution whose density is 0.003 to 0.03% on the SiO-deposited substrate and sintering the resulting substrate at a temperature of 80.degree. C. The UTPFE film can be formed on the SiO-deposited substrate by introducing gaseous monomer to a vacuum device and performing a RF (Radio Frequency) plasma discharge between tabular electrodes provided inside of the vacuum device. When the deposited substrate with the DMOAP formed thereon is formed to be cellular and cyanobiphenyl liquid crystal, CB1, is sealed in the cell, the angle of .theta.p becomes .theta.p=16.degree. to 26.degree.. Further, the substrate with the UTPFE film formed on the SiO material is formed to be cellular and CB7 is sealed into the cell, the angle of .theta.p becomes .theta.p=22.degree. to 30.degree.. Further, when azoxy liquid crystal, HXAB, is sealed in the cell, the orientation is not tilted but completely vertical, however, the orientation changes with time. This paper does not have any description about light stability. In the next example, by coating a PTFE (polytetrafluoroethylene) film on the SiO surface whose SiO is deposited at an angle of o=60.degree., the pre-tilt can be obtained (L. Rousille and J. Robert Applied Physics Letter 50 p. 3975 1979). Since the angle of .theta.p depends on a film thickness of the PTFE film, the thickness is optimized. In the case of d=20.ANG., the angle of .theta.p=8.degree. can be obtained. Though this cell is left in room temperature for several months, no change takes place in the orientation of the liquid crystal molecules. This paper does not have any description about light stability. As another method for keeping the vertical orientation of the liquid crystal molecules tilted, it has been reported to take the steps of mixing two kinds of silane coupling agent, MAP (N methylaminopropyltrimethoxysilane: to obtain a parallel orientation) and OTS (octadecyltriethoxysilane: to obtain a vertical orientation) at a certain ratio, diluting the mixed material with IPA (isopropyl alcohol), performing an orientating treatment on the glass substrate with the diluted solution, and then rubbing on the substrate (Robert W. Filas and J. S. Patel Applied Physics Letter 50 p. 1428 1987). In this method, by optimizing the rubbing strength, the angle of .theta.p=3.degree. to 4.degree. can be obtained. However, some disadvantages are referred in the paper. For example, the silane coupling agent solution changes with time and the orientation is not stable at a high temperature. This paper does not have any description about light stability. Further, as a method for obtaining a vertical orientating state, it has been reported to execute the orientating treatment by using a chromic chain (tetrachloro--.mu.--hydroxo--.mu.--carboxylatodichromium (III) complex) (Shoichi Matumoto et, al. Applied Physics Letter 27 p. 268 1975). The liquid crystal cell at a dynamic scattering mode produced by this method keeps a stable orientation for maximum 1500 hours at the driving aging test (which is a reliability test for applying alternate current of 50 Hz and 30 Vrms to the liquid crystal cell at a temperature of 60.degree. C). However, this paper does not have any report about light stability.
Moreover, as a method for keeping the vertical orientation of the liquid crystal molecules tilted, it has been reported to obliquely depositing the material of SiO.sub.x (x=1, 2) and perform the finishing treatment with alcohol (Patent Lying Open No. Hei 2-508482, applied by Hughes Aircraft Company). The oblique deposition of SiO.sub.x is divided into two phases. At the first phase, SiO.sub.x is deposited at an angle of o=20.degree. to 40.degree. (preferably, 30.degree.). At the second phase, the substrate is rotated 90.degree. and the deposition is done at an angle of o=2.degree. to 10.degree. (preferably, 5.degree.). In succession, by forming a surface alkoxy on the substrate by using the response to a long chained alcohol with a surface hydroxyl group, the tilted vertical orientation is allowed to be obtained. There is a description that the tilted vertical orientation obtained by this method indicates a light stability. However, since the dielectric anisotropy of the crystal liquid used in this paper is positive, the resulting angle of .theta.p is so large a value as .theta.p=6.degree. to 45.degree.. Hence, in this method, it is difficult to obtain a fine pre-tilt angle. Moreover, the method for rubbing process on a vertically oriented polyimide film for obtaining a pre-tilt angle is applied to the liquid crystal display element in which a liquid crystal mixed with a chiral agent and a dichromatic dye are sealed (Japanese Patent Lying No. Hei 3-107925, applied by Stanley Electrics). In this paper, the display mode of the liquid crystal layer is limited as a guest host mode.
To project a full-color display, it is necessary to take the steps of dividing a white light into three colors, red, blue and green, forming an image about each of the colors, synthesizing the formed images, and expansively projecting the synthesized image through the effect of the optical system. As mentioned above, the HFE mode provides dependency of a wavelength on the reflectance characteristic at the applied voltage is off time. In designing a panel, it is possible to define parameters so as to meet the aforementioned conditions. However, it is a satisfactory condition to only a simple wavelength. The wavelength band of the divided ray is as broad as 100 nm. Hence, the dominant wavelength does not meet the aforementioned condition. Hence, the actual reflectance at the time of OFF is not disadvantageously made zero. This disadvantage puts a restriction to obtaining a high contrast at the HFE mode.
At the HFE mode, as shown in FIG. 4, a threshold voltage depends on the temperature too much. In the projection, it is originally likely that the panel is heated up to a temperature which is close to a reading light source. However, according to the continuous use of the projection system, the illumination of the light source is degraded. The temperature of the panel is made lower accordingly, so that the threshold voltage of the liquid crystal may be shifted to the high voltage side. Hence, the threshold voltage is shifted out of the optimized driving voltage, resulting in remarkably lowering the display quality.
FIGS. 5 and 6 show a voltage to reflectance (V-R) characteristic in the liquid crystal light valve at the HFE mode. FIG. 5 shows the V-R characteristic in the case of a large on-off ratio. FIG. 6 shows the V-R characteristic in the case of a small on-off ratio. Herein, the description will be oriented to a parameter of an on-off ratio. The peak of the V-R characteristic exists on the side of a high applied voltage if the writing ray of light does not come into the photoconductive layer (in the dark state) or on the side of a low applied voltage (in the photo state) if the writing ray of light comes into the photoconductive layer. The on-off ratio is defined by the following expression. EQU on/off=V.sub.Rmax dark /V.sub.Rmax photo
where V.sub.Rmax dark : applied voltage given when a maximum reflectance is obtained in the dark state and V.sub.Rmax photo : applied voltage given when the maximum reflectance in the photo state.
The HFE mode is inferior in acuteness of the reflectance characteristic. As shown in FIG. 6, hence, if the on-off ratio of the panel is low, no bright display cannot be disadvantageously obtained. The DAP type ECB mode is characterized in that it is superior in acuteness of the voltage to reflectance characteristic to the HFE mode and the perfect black can be displayed when no voltage is applied in the crossed-Nicol state.
However, the conventional method proposed for realizing the tilted vertical orientation required for the DAP type ECB mode has the following disadvantages. With the oblique deposition method, the pre-tilt angle of the liquid crystal greatly changes if the deposition conditions such as a deposition angle, a deposition speed, a vacuum level, a substrate temperature and a film thickness change or the liquid crystal material or the deposition material changes. Further, for the large substrate, it is very difficult to make the conditions equal with each other on the plane. Hence, it is difficult to obtain a uniform tilted vertical orientation on a large area with excellent reproducibility.
If the aforementioned conventional orientating method may apply to the projection, the sufficient light stability is required. However, up to now, no data about the light stability has been reported.